Hitherto, there has been known a vehicle brake control device configured to generate a braking torque by an electric motor. In this type of device, typically, an indicator current (target current) is calculated based on an operation amount of a braking operation member for the vehicle by a driver, and the electric motor is controlled based on the indicator current. Then, a braking torque that depends on the operation of the braking operation member is applied to the wheels.
In the device of this type, due to influences of the inertia (inertia moment, inertia mass) of the entire device including the inertia of the electric motor, particularly in a case of abrupt braking (upon rapid increase of braking torque) or the like, a response delay of the braking torque (a delay of rise thereof) may occur in acceleration during which the rotation speed of the electric motor is increased (for example, when the electric motor is started) and overshoot of the braking torque may occur in deceleration during which the rotation speed of the electric motor is decreased (for example, when the electric motor is intended to be stopped). Therefore, particularly, at abrupt braking, it is desired to compensate for the influences of the inertia, that is, to improve responsiveness (rising performance) of the braking torque during acceleration of the electric motor and to suppress the overshoot of the braking torque during deceleration of the electric motor (improve the convergence).
In order to cope with this problem, for example, Japanese Patent Application Laid-open No. 2002-225690 has the following description. That is, based on a map in which indicator currents and target motor rotation angles are defined, a target motor rotation angle corresponding to the calculated indicator current is determined, and a target motor rotation angular acceleration is calculated by subjecting the target motor rotation angle to the second-order differentiation. Based on the target motor rotation angular acceleration, an inertia compensation current for compensating for the influences of the inertia of the entire device is calculated. In this case, the inertia compensation current is calculated to be a positive value during acceleration of the electric motor, while the inertia compensation current is calculated to be a negative value during deceleration of the electric motor. This inertia compensation current is added to the indicator current, to thereby determine a compensated indicator current (target current). In this manner, the compensated indicator current is calculated to be slightly larger than the indicator current when the electric motor is started so that the responsiveness of the braking torque can be improved. The compensated indicator current is calculated to be slightly smaller than the indicator current when the electric motor is intended to be stopped so that the overshoot of the braking torque can be suppressed.
In addition, Japanese Patent Application Laid-open No. 2002-225690 also describes providing a “gradient limitation” against the indicator current for performing stable control when the indicator current exceeds the capacity of the electric motor. Specifically, by providing a gradient restriction in advance so that at least an gradient of the indicator current does not exceed the capability of the electric motor, the gradient of the indicator current and an actual output value are made to coincide with each other. Alternatively, when the actual output value cannot follow the initially set indicator current, the indicator current and the actual output value are compared to each other. Based on the result of comparison, the gradient of the indicator current is restricted. In this case, a map which defines the relationship between “the indicator current and the actual output value” and “the gradient restriction of the indicator current” is created in advance. Based on the map, the gradient restriction of the indicator current is obtained based on the map from the above-mentioned result of comparison.